1. Field of the Invention
The present invention relates to a high-temperature ceramic heater used as an ignition heater for various combustion apparatuses and also used as a heater for various heating apparatuses.
2. Prior Art
A sheath heater or the like with a heating resistor comprising heat-resistant insulating powder and a metal wire with a high melting point embedded in a heat-resistant metal sheath has been used as an ignition heater for various combustion apparatuses which burn gas and kerosene and has also been used as a heater for various heating apparatuses.
The above-mentioned sheath heater, however, has some defects. It cannot exhibit quick temperature rising characteristics since heat is transmitted via the heat-resistant insulation powder. It is inferior in oxidation resistance and durability. In addition, it lacks reliability for positive ignition and raises problems in safety.
To solve such problems, a ceramic heater comprising a heating resistor of an inorganic conductor embedded in a ceramic sintered body, which can quickly raise temperature, can be used for an extended period of time regardless of environmental conditions and is superior in ignition reliability and safety, has been used widely as an ignition source for various combustion and heating apparatuses.
The most popular type of such a ceramic heater is featured such that silicon nitride superior to other ceramic materials in thermal shock resistance and high-temperature strength is used as a heater substrate and a heating resistor made of a metal with a high melting point such as tungsten (W) or molybdenum (Mo) or a compound of these metals is embedded in the heater substrate, or a ceramic heater wherein heating resistor paste mainly composed of one of the above-mentioned metals with high melting points or a compound of such metals is pattern-printed on the above-mentioned substrate and sintered into one body.
However, a ceramic heater wherein a conventional silicon nitride sintered body is used as the substrate causes deterioration in the strength and structure of the silicon nitride sintered body, since a glass component comprising alumina (Al.sub.2 O.sub.3), magnesia (MgO), calcia (CaO), etc. and remaining in the grain boundaries of the sintered body is softened when the temperature of the heater is raised over 1,000.degree. C. by electric heating. This deterioration generates cracks in a part of the silicon nitride sintered body adjacent to the heating resistor and lowers oxidation resistance.
To eliminate the above-mentioned defects, a ceramic heater 12 has been proposed wherein a main heating element 10 made of titanium nitride or vanadium nitride, being as conductive as metals, is built in a silicon nitride sintered body 11 which is superior in mechanical property, heat resistance, thermal shock resistance and chemical stability at high temperatures, as shown in FIG. 3 (Japanese Patent Publication No. 62-59858).
However if the above-mentioned ceramic heater 12 is used to ignite kerosene or oil fan heater or to generate high temperatures exceeding 1,300.degree. C., the ceramic heater is required to have durability at such high temperatures exceeding 1,300.degree. C. to achieve positive ignition, stability and reliability. In particular, when the ceramic heater 12 is used to ignite various gas combustion apparatuses, the heater is requested to have even higher heat resistance at temperatures exceeding 1,400.degree. C. and quick temperature rising characteristics represented by an ignition temperature reaching period of not more than 3 seconds. In an electricity application cycle test wherein each cycle consists of a quick temperature rising period during which 1,500.degree. C. is reached within 10 seconds after AC voltage regulated to saturate at 1,500.degree. C. for example is applied and a constant electricity application stop period, the ignition heater used for this test is required to withstand 10,000 cycles. The above-mentioned ceramic heater 12 cannot satisfy the specified characteristics for the electricity application cycle test. The main heating element 10 of the ceramic heater 12 may be broken or its resistance may be changed. In addition, the silicon nitride sintered body 11 itself, used as the substrate of the ceramic heater 12, tends to oxidize. For these problems, the above-mentioned ceramic heater 12 lacks in durability and reliability when used as a source for ignition and heating at high temperatures. Furthermore, the ceramic heater 12 cannot shorten the time required to reach 1,000.degree. C. to 5 seconds or less.